Multi-element composite object

ABSTRACT

A multi-element composite object composed from first, second, and third metal components is provided, wherein the first metal and the third metal are weld incompatible. The multi-element composite object includes a first component fabricated from a first metal. A second component, fabricated from a second metal, is brazed to the first component. A third component, fabricated from a third metal, is inertia welded to the second component. The first metal may be provided as a titanium alloy, e.g. a TiNi alloy. The second metal may be provided as low-carbon mild or alloy steel. The third metal may be provided as alloy steel, e.g., 9310 nickel alloy steel. In an embodiment, the multi-element composite object is a gear assembly, with the first element of the gear assembly object being a shaft and the third element of the gear assembly being a gear member with hardened teeth surfaces. The first and second components can be mechanically keyed together via an anti-rotational element. The anti-rotational element can be provided as a pin-in-groove arrangement or a twist-fit arrangement. A method of making a multi-metal composite object including a first component fabricated from a first metal, a second component fabricated from a second metal, and a third component fabricated from a third metal, wherein the first metal and the third metal are weld incompatible, is also disclosed. The first step of the method includes mechanically keying the first component to the second component. Next, the first component is brazed to the second component. Finally, the third component is welded to the second component. Where the first metal is a Ti alloy and the second metal is low-carbon steel, the step of brazing the first component to the second component can include brazing using a brazing material such as Ag and Cu. Where the third component is heat-treated steel, the assembly can be stress-relieved after inertia welding at a temperature sufficiently low so as not to degrade the heat-treated properties of the third component.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] None

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSOREDRESEARCH AND DEVELOPMENT

[0002] None

FIELD OF THE INVENTION

[0003] The invention relates generally to multi-element compositeobjects. In particular, the invention relates to multi-element compositeobjects fabricated from weld-incompatible metals.

DESCRIPTION OF RELATED ART

[0004] Objects employing combinations of different metals, so- called“composite objects” are hardly new. For example, archaeologists havediscovered artifacts combining iron and bronze components that date frombefore the birth of Christ. One basic premise present in these primitivemetallurgical developments has carried through the centuries thedesirable characteristics of different metals may be combined by theiruse in a single object.

[0005] One modern application of this premise is in the aircraftindustry. It is well-recognized that every gram of weight that can beremoved from an aircraft will pay large dividends by way of reduced fuelconsumption, increased performance, or increased payload. Thus, aconstant theme in the manufacture of aircraft components is the need toreduce the weight of every component while maintaining or increasingcomponent strength and structural integrity.

[0006] One area in which this theme is illustrated abundantly is in thefabrication of gears for use in aircraft. It is typical of suchstructures that the gear teeth, splines, and bearing races call formaterials that have hard surfaces to resist wear, contact fatigue, andbending fatigue. By contrast, gear web and shaft portions are free ofsuch requirements, and are therefore prime candidates for achievingweight reduction.

[0007] In order to meet these goals, steel alloy gears with titaniumalloy webs and shafts have been proposed. Unfortunately, traditionalwelding and casting methods are practically ineffective for joiningthese alloys together. One solution to this problem is described in U.S.Pat. No. 5,492,264 to Wadleigh et al., which is incorporated byreference herein. The '264 disclosure is directed to a composite objectformed by using inertia welding to join first and second dissimilarelements together via a third, mutually-compatible interlayer metalelement. The method for forming the composite object involves inertiawelding the interlayer to the first element, then inertia welding thesecond element to the interlayer. In an illustrative embodiment, thecomposite object is a multi-metal element composite gear, web, andshaft. In a preferred embodiment, a hardened steel alloy gear is inertiawelded to an aluminum interlayer, which is in turn inertia welded to atitanium alloy shaft.

[0008] While this solution overcomes many of the longstanding problemsdescribed herein, it is not without possible shortcomings itself. Onearea for improvement is in the nature of the interlayer. Despite thefact that aluminum is used extensively in the aircraft industry, thereis an impression of vulnerability associated with some aluminumcomponents. This impression is that, while aluminum is a very desirablematerial for airframes, it is unsuitable for powerplant and transmissionapplications. Aluminum components introduce a temperature limitation ofapproximately 350° F., above which the metal begins to thermally soften.Although gearboxes do not ordinarily operate at temperatures above 350°F., there is a military requirement for gearboxes to function for onehour after all the oil is drained out. The reason for this damagetolerance is to allow the crew sufficient time for escape and egressafter the gearbox has been punctured, typically by gunfire or otherordnance. In the civilian world, it is extremely rare for aircraft to besubjected to gunfire, even when such aircraft are operating overhigh-crime urban areas. However, particularly in the case ofhelicopters, the same manufacturers make aircraft for civilian andmilitary use. Consequently, the design approach (and many standardcomponents such as gearboxes) are common to both applications.

[0009] It can be seen from the foregoing that the need exists formulti-metal composite objects, and methods for their manufacture, thatmeet weight and strength objectives without sacrificing emergencyoperating capabilities.

SUMMARY

[0010] The present invention achieves these and other objects byproviding a multi-element composite object composed from first, second,and third metal components, wherein the first metal and the third metalare weld incompatible. The multi-element composite object includes afirst component fabricated from a first metal. A second component,fabricated from a second metal, is brazed to the first component. Athird component, fabricated from a third metal, is inertia welded to thesecond component.

[0011] The first metal may be provided as a titanium alloy, e.g. a TiNialloy. The second metal may be provided as steel, e.g., low-carbon alloyor mild steel. The third metal may be provided as alloy steel, e.g.,9310 nickel alloy steel.

[0012] In an embodiment, the multi-element composite object is a gearassembly, with the first element of the gear assembly object being ashaft and the second element of the gear assembly being a gear memberwith hardened teeth surfaces. The first and second components can bemechanically keyed together via an anti-rotational element. Theanti-rotational element can be provided as a pin-in-groove arrangementor a twist-fit arrangement.

[0013] A method of making a multi-metal composite object including afirst component fabricated from a first metal, a second componentfabricated from a second metal, and a third component fabricated from athird metal, wherein the first metal and the third metal are weldincompatible, is also disclosed. The first step of the method includesmechanically keying the first component to the second component. Next,the first component is brazed to the second component. Finally, thethird component is welded to the second component. Where the first metalis a Ti alloy and the second metal is low-carbon mild or alloy steel,the step of brazing the first component to the second component caninclude brazing the steel component to the Ti alloy component using abrazing material selected from a group consisting of Ag and Cu. Wherethe third component is heat-treated steel, the inertia weld jointbetween the second and third components may be stress-relieved at atemperature sufficiently low so as not to degrade the heat-treatedproperties of the third component after inertia welding the thirdcomponent to the second component.

[0014] The features of the invention believed to be patentable are setforth with particularity in the appended claims. The invention itself,however, both as to organization and method of operation, together withfurther objects and advantages thereof, may be best understood byreference to the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic sectional view of a multi-metal compositeobject.

[0016]FIG. 2 illustrates an elevational view of a multi-metal compositegear assembly.

[0017]FIG. 3 is an exploded view of the gear assembly shown in FIG. 2.

[0018]FIG. 4 is a sectional view taken along lines IV-IV of FIG. 2.

[0019]FIG. 5 illustrates an elevational view of a multi-metal compositegear assembly.

[0020]FIG. 6 is an exploded view of the gear assembly shown in FIG. 5.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0021] While the invention is susceptible of embodiment in manydifferent forms, there are shown in the drawings, and will herein bedescribed in detail, exemplary embodiments, with the understanding thatthe present disclosure is to be considered as illustrative of theprinciples of the invention and not intended to limit the invention tothe exemplary embodiments shown and described.

[0022] A multi-metal composite object 10 is shown in FIG. 1. Themulti-metal composite object 10 is illustrated as what is commonly knownas a torsion test coupon. The multi-metal composite object 10 includes afirst component 12, a second component 14, and a third component 16. Thefirst component 12 is fabricated from a first metal selected to providea desired characteristic, for example, the component 12 may befabricated from a titanium alloy, such as Ti-6Al-4V, to provide reducedweight. The third component 16 is fabricated from a second metalselected to provide a different desired characteristic, for example, thecomponent 16 may be fabricated from a steel alloy, such as carburizedalloy 9310, to provide strength and surface hardness.

[0023] It is frequently the case that metals having widely differingcharacteristics have molecular compositions that render them difficultto join together, i.e., weld incompatible. This is true with titaniumalloys and steel alloys generally, and with Ti alloy Ti-6Al-4V andcarburized steel alloy 9310 specifically. Accordingly, the secondcomponent 14 is provided as a connecting member between the firstcomponent 12 and the third component 16. The second component 14 isfabricated from a material that can be joined to the weld-incompatiblematerials of the first component 12 and the third component 16. In thepresent example, the second component 14 can be fabricated from alow-carbon mild steel or low-carbon alloy steel, such as 1018 or 9310steel. It is believed that alloy steel or mild steel having a carboncontent below around 0.25 would be suitable.

[0024] As shown in FIG. 1, the first component 12 is joined to thesecond component 14 by brazing, for example, by vacuum/inert gas brazingwith silver alloy or copper alloy brazing material. In order to provideadditional rotational strength, one or more keys, here shown ascross-sectionally square steel keys 17, may be provided. The keys 17 fitinto corresponding grooves 18, 19 in the first component 12 and thesecond component 14. The first component 12, second component 14, andkeys 17 are assembled, and then brazed completely along all interfaces.If the first and second components were not mechanically keyed together,the torsional operating stresses would be borne by the braze materialalone. The provision of mechanical keying causes the torsional operatingstresses to be at least partially transferred to the keys themselves,through which the load passes.

[0025] Once the first component 12 is joined to the second component 14,a low-carbon steel to alloy steel weld is performed between the secondcomponent 14 and the third component 16. This weld is performed inaccordance with the inertia welding and friction welding set forth inU.S. Pat. No. 5,492,264. The carbon content of the second component 14must be low enough so as to avoid the formation of brittle, untemperedmartensite in the weld region.

[0026] Unfavorable metallurgical changes may occur to the titanium alloyat approximately 1800° F. In order to avoid such changes, the brazingstep is carried out at a temperature slightly lower than 1800° F. Forexample, if silver alloy braze is used, brazing is performed at amaximum temperature of 1700° F., which then becomes the temperaturelimitation of the component. This compares rather favorably with the350° F. limitation associated with the aluminum interlayer multi-metalcomposite gear technology. The elevated brazing temperature does,however, produce an annealed or normalized structure in the low-carbonsteel connecting member.

[0027] Although the lower strength condition of the intermediate steelcomponent would be of inadequate hardness for a gear or bearing surface,the composite component as described is more than adequate as astructural member. The large load path cross-section results in stressamplitudes in the intermediate component that are well within safelevels for non-heat-treated materials.

[0028] Except in a very localized region of the weld, the inertia welddoes not adversely affect the properties of the heat-treated thirdcomponent 16. Consequently, the third component 16 can be quenched andtempered (and any features formed thereon, such as gear teeth, can beformed and gas carburized), prior to the welding operation. The treatedthird component 16 would remain unaffected, and retain the advantages ofhaving been heat-treated, during subsequent service of the multi-metalcomposite object 10 . As an additional measure, the assembly can beheated to around 300° for several hours in order to relieve anylocalized internal stresses that may be caused by inertia welding. Thisstress-relief operation does not adversely affect the heat-treatedproperties of the carburized features (such as bearing races or gearteeth) on the third component 16.

[0029] Turning now to FIG. 2, a gear assembly 20 is shown which embodiesthe principles of the present invention. The gear assembly 20 includes ashaft 22 fabricated from a titanium alloy, such as Ti-6Al-4V. Disposedat each end of the shaft 22 is a gear member 24 having a plurality ofgear teeth 26.

[0030] As can be seen in FIG. 3, a connecting member 28 is providedbetween the shaft 22 and each gear member 24. The connecting member 28is provided with grooves 30 which correspond in number and configurationwith grooves 32 provided on a cylindrical extension 34 of an end of theshaft 22. The grooves 30, 32 are adapted to receive keys 36 when thesubassembly including shaft 22, intermediate connecting member 28, andkeys 36 are assembled prior to brazing. The connecting member 28 is alsoprovided with a frustoconical surface 38, which corresponds inconfiguration to a frustoconical surface 40 on the gear member 24, asshown in FIG. 4.

[0031] Fabrication of the gear assembly 20 may be understood by those ofskill in the art with reference to the exploded view illustrated in FIG.3 and the cross-sectional view of FIG. 4. The connecting member 28 andthe extension 34 of the shaft 22 are brought together with the grooves30, 32 aligned, and the keys 36 are inserted therein. Next, thissubassembly is brazed completely along all interfaces between theconnecting member 28, the shaft 22, and the keys 36. Finally, thefrustoconical surface 38 of the connecting member 28 is welded to thefrustoconical surface 40 of the gear member 24 by inertia welding orfriction welding.

[0032] Turning next to FIG. 5, a gear assembly 42 is shown which alsoembodies the principles of the present invention. The gear assembly 42includes a shaft 44 fabricated from a titanium alloy, such as Ti-6Al-4V.Disposed at each end of the shaft 44 is a gear member 46 having aplurality of gear teeth 48.

[0033] As can be seen in FIG. 6, a connecting member 50 is providedbetween the shaft 44 and each gear member 46. The connecting member 50is provided with grooves 52 which correspond in number and configurationwith grooves 54 on a cylindrical extension 56 on an end of the shaft 44.The grooves 52, 54 are adapted to receive keys 58 when the gear assembly42 is assembled. Additionally, the connecting member 50 is provided witharcuate cam slots 60, which extend from the grooves 52 and are adaptedto receive cam followers 62 on the cylindrical extension 56 in a“bayonet” connection. The connecting member 50 is also provided with afrustoconical surface 64, which corresponds in configuration to afrustoconical surface 66 on the gear member 46.

[0034] Fabrication of the gear assembly 42 is as follows. The connectingmember 50 and the extension 56 of the shaft 44 are brought together withthe cam followers 62 inserted into the grooves 52. The connecting member50 and the shaft 44 are then rotated relative to one another, such thatthe cam followers 62 travel along the arcuate cam slots 60, and thegrooves 52 are brought into alignment with the grooves 54. Next, thekeys 58 are inserted into the aligned grooves 52, 54, and thesubassembly is brazed completely along all interfaces between theconnecting member 50 (including the cam followers 62), the shaft 44(including the cam slots 60), and the keys 58. Finally, thefrustoconical surface 64 of the connecting member 50 is welded to thefrustoconical surface 66 of the gear member 46 by inertia welding orfriction welding.

[0035] Thus it is apparent that in accordance with the presentinvention, an apparatus that fully satisfies the objectives, aims, andadvantages achievable in accordance with the principles of the presentinvention is set forth in the above exemplary embodiments. The presentinvention achieves the weight and strength of previously knowntechnologies while allowing the gear assembly to operate for meaningfulperiods of time at temperatures in excess of 1000° F. The process ofjoining the elements of the multi-metal composite object is accomplishedsuch that service abuse limitations are those associated with the gearteeth, rather than the joining process itself, as is the case withsingle-material assemblies. This is accomplished while achievingsignificant weight reductions, perhaps in the order of 25-30%, dependingon the particular component geometry and specifications.

[0036] While the invention has been described in conjunction with theexemplary embodiments, it is evident that many alternatives,modifications, permutations, and variations will become apparent tothose skilled in the art in light of the foregoing description. Forexample, the locations of the cam followers and slots could be reversed,or other materials used for brazing, or for the components themselves.It is also conceivable that the keying mechanisms could be eliminatedentirely. The present invention could also find utility in applicationsother than gear assemblies, for example, in other rotational drivemembers. Accordingly, it is intended that all such alternatives,modifications, permutations, and variations to the exemplary embodimentscan be made without departing from the scope and spirit of the presentinvention as set forth in the appended claims.

1. A multi-element composite object comprising the following a firstcomponent fabricated from a first metal; a second component, fabricatedfrom a second metal, brazed to the first component; and a thirdcomponent, fabricated from a third metal, inertia welded to the secondcomponent; wherein the first metal and the third metal are weldincompatible.
 2. A multi-element composite object according to claim 1,wherein the first metal is a titanium alloy.
 3. A multi-elementcomposite object according to claim 2, wherein the first metal is a TiNialloy.
 4. A multi-element composite object according to claim 1, whereinthe second metal is steel.
 5. A multi-element composite object accordingto claim 4, wherein the second metal is low-carbon steel.
 6. Amulti-element composite object according to claim 1, wherein the thirdmetal is a steel alloy.
 7. A multi-element composite object according toclaim 6, wherein the third metal is 9310 nickel alloy steel.
 8. Amulti-element composite object according to claim 1, wherein themulti-element composite object is a gear assembly.
 9. A multi-elementcomposite object according to claim 8, wherein the first element of thegear assembly object is a shaft.
 10. A multi-element composite objectaccording to claim 8, wherein the third element of the gear assembly isa gear member.
 11. A multi-element composite object according to claim9, wherein the third element of the gear assembly is a gear member withhardened teeth surfaces.
 12. A multi-element composite object comprisingthe following a titanium alloy component; a low-carbon steel componentbrazed to the titanium alloy component; and an alloy steel componentinertia welded to the low-carbon steel component.
 13. A multi-elementcomposite object according to claim 12, further comprising ananti-rotational mechanism connected between the Ti alloy component andthe low-carbon steel component
 14. A multi-element composite objectaccording to claim 13, wherein the anti-rotational element comprises apin-in-groove arrangement.
 15. A multi-element composite objectaccording to claim 13, wherein the anti-rotational element comprises atwist-fit arrangement.
 16. A multi-element composite object according toclaim 12, wherein the multi-element composite object is a gear assembly.17. A method of making a multi-metal composite object including a firstcomponent fabricated from a first metal; a second component fabricatedfrom a second metal, and a third component fabricated from a thirdmetal, wherein the first metal and the third metal are weldincompatible, the method comprising the following steps mechanicallykeying the first component to the second component; brazing the firstcomponent to the second component; and welding the third component tothe second component.
 18. A method according to claim 17, wherein thefirst metal is a Ti alloy, the second metal is low-carbon steel, and thestep of brazing the first component to the second component comprisesbrazing the mild steel component to the Ti alloy component using abrazing material selected from a group consisting of Ag and Cu.
 19. Amethod according to claim 17, wherein the third component isheat-treated and further comprising the step of stress- relieving thethird component at a temperature sufficiently low so as not to degradethe heat-treated properties of the third component.
 20. A methodaccording to claim 19, wherein the step of of stress-relieving the thirdcomponent is performed after the step of inertia welding.